科研之友
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摘要
The relationships between the intrusion of gneissose granitoids and the attainment of regional high-T conditions recorded in metamorphic rocks from the Ryoke belt of the Mikawa area, central Japan, are explored. Seven gneissose granitoid samples (tonalite, granodiorite, granite) were collected from three distinct plutonic bodies that are mapped as the so-called" Older Ryoke granitoids." Based on bulk-rock compositions and U-Pb zircon ages obtained by laser ablation inductively coupled plasma mass spectrometry, the analyzed granitoids can be separated into two groups. Gneissose granitoids from the northern part of the area give weighted mean 206Pb/238U ages of 99 +/- 1 Ma (two samples) and 95 +/- 1 Ma (one sample), whereas those from the southern part yield 81 +/- 1 Ma (two samples) and 78-77 +/- 1 Ma (two samples). Regional comparisons allow correlation of the northern granitoids (99-95 Ma) with the Kiyosaki granodiorite, and mostly with the Kamihara tonalite found to the east. The southern granitoids are tentatively renamed as "78-75 Ma (Hbl)-Bt granite" and "81-75 Ma Hbl-Bt tonalite" (Hbl, hornblende; Bt, biotite). and seem to be broadly coeval members of the same magmatic suite. With respect to available age data, no gneissose granitoid from the Mikawa area shows a U-Pb zircon age which matches that of high-T metamorphism (ca 87 Ma). The southern gneissose granitoids (81-75 Ma), although they occur in the highest-grade metamorphic zone, do not seem to represent the heat source which produced the metamorphic field gradient with a low dP/dT slope.
- 出版日期2018-1
